A joist is a single solid member, a sawn 2×10 or an engineered I-joist, that spans between supports and carries a floor or ceiling by bending. A truss is a factory-built frame of smaller lumber pieces joined in triangles with steel plates, designed by the manufacturer for one specific span and load, and it can span farther with less material but cannot be cut or altered on site.
Both are used for floors and roofs. This guide compares how they work, where each makes sense, and what each means for remodeling, mechanical runs and attic space later on.
How a Joist Works
A joist is a beam in a repeating pattern. It sits on bearing at each end (a wall, beam or ledger), and the load on top makes it bend. The top edge is squeezed, the bottom edge is stretched, and the depth of the member is what gives it bending strength and stiffness. Floor joists, ceiling joists and rafters all work this way. If the difference between a joist and the beams that hold it up is unclear, see girder vs beam vs joist.
Joists are either sawn lumber (2×6 to 2×12) or engineered wood I-joists, which use solid flanges top and bottom and a thin OSB web between them. HUD’s Residential Structural Design Guide, Chapter 5 notes that floors have traditionally used solid sawn joists, with parallel chord trusses and I-joists in increasing use for their dimensional consistency and longer spans.
How a Truss Works
A truss replaces one deep solid member with a frame: a top chord, a bottom chord and web members between them arranged in triangles. A triangle cannot change shape unless one of its sides changes length, so the load is carried mostly as tension and compression along each piece instead of bending across a solid section. The pieces meet at panel points, joined by galvanized or stainless steel plates whose teeth are pressed into the wood.
That arrangement lets a truss use small lumber (usually 2×4 or 2×6 for roof trusses, according to HUD) over long spans. HUD notes that roof trusses can span the full width of a home without interior support walls, which frees up interior layout.
Truss vs Joist at a Glance
| Joist (sawn or I-joist) | Truss | |
|---|---|---|
| Made | Lumber yard stock or engineered product, cut to length on site | Built in a plant to a design for that building |
| Designed by | Code span tables or manufacturer span tables | Truss manufacturer’s designer, from loads and geometry the building designer supplies |
| How it carries load | Bending of a solid section | Tension and compression in a triangulated frame |
| Span | Limited by lumber depth; I-joists go farther | Long spans with small lumber; little or no interior bearing needed |
| Mechanical runs | Holes and notches only within strict limits | Open webs let pipes, wires and some ducts pass through |
| Field changes | Can be cut to length and headered around openings | Not cut or altered without the truss designer’s or an engineer’s approval |
| Attic or floor space | Rafters plus ceiling joists leave an open attic | Standard roof trusses fill the attic with webs (attic trusses exist) |
| Lead time | Off the shelf | Must be designed and fabricated before delivery |
Roofs: Trusses vs Rafters and Ceiling Joists
On a roof, “joist” usually means the ceiling joists that pair with rafters in stick framing. HUD describes a roof truss as combining the rafters (top chords) and ceiling joists (bottom chords) into one frame with web members between them. Stick framing builds that same assembly on site one piece at a time, and the ceiling joists or other cross-ties hold the walls against the rafters’ outward thrust. Without them, the roof needs a structural ridge beam; see ridge beam vs ridge board.
The Structural Building Components Association’s roof truss overview (checked October 2026) lists the usual advantages: buildings get enclosed faster, there is less cutting waste, and exterior walls and roofs can often go up without interior bearing partitions. The trade-offs are attic space, since standard trusses fill it with webs unless you order attic or scissor trusses, and flexibility: trusses are designed for one building and, as SBCA puts it, generally cannot easily be reused elsewhere.
Rafters still make sense for complex roofs with many hips, valleys and dormers, small additions, vaulted ceilings, and jobs where the attic will become living space. HUD notes it is still common to use rafter framing in the more complex parts of roofs that are otherwise trussed.
Floors: Floor Trusses vs Floor Joists
Floor trusses are parallel-chord trusses, usually built from 2×4 or 2×3 lumber laid flat. According to SBCA’s floor truss page (checked October 2026), the flat chords give a wide nailing surface for glue and sheathing, the open web leaves room for plumbing, electrical and mechanical runs, trusses can be designed with a chase for ductwork running across the truss layout, and most floor truss machines can build trusses up to 24 inches deep.
Solid sawn joists remain the simple choice for short spans, repairs, additions and anything that will be framed and adjusted on site. Engineered I-joists sit between the two: they are cut to length on site like lumber but span farther, and they allow web holes within the manufacturer’s size and location rules. Our wood I-beam sizes page lists common I-joist depths.
Who Designs Each One
Lumber joists in ordinary houses are usually picked from the building code’s span tables, and I-joists from the manufacturer’s span tables. Trusses are different: HUD explains that the truss manufacturer normally provides the engineering design, and the building designer has to give the manufacturer the design loads, truss profile, support locations and any special requirements. The truss design drawings that come with the delivery show the loads, the bearings and which web members need permanent lateral bracing.
Truss bracing during installation matters for safety. HUD notes that temporary bracing during construction is usually the contractor’s responsibility, and SBCA points installers to its BCSI guide for bracing requirements. Trusses longer than 60 feet need special installation and bracing.
What Each Means for Remodeling
This is where the choice shows up years later.
- Cutting. A joist can be cut and headered around a new stair or skylight by following code rules or an engineer’s detail. A truss member cannot be cut, notched, drilled or removed without approval from the truss designer or a licensed structural engineer, because every piece is part of a designed frame. Repairs to damaged trusses use engineered repair details.
- Attic conversions. Rafter-framed attics can sometimes be finished with upgraded ceiling joists. A standard trussed attic cannot be turned into a room by removing webs.
- Interior walls. Because roof trusses often span wall to wall, interior walls under them may not be load-bearing, but that is not guaranteed: girder trusses and interior bearing points exist. Confirm before removing any wall; see our load-bearing wall removal guide.
- Loads. Adding a heavy load, such as storage platforms, HVAC equipment or solar panels, to trusses needs a check against the truss design.
Steel construction has the same split. Open-web steel bar joists are trusses by structure even though they are called joists, while rolled W-shape beams are solid members. The idea is the same: triangulate to span far with little material, or use a deeper solid section that is easier to modify.
Truss vs Joist FAQ
Is a truss stronger than a joist?
Neither is stronger in general. Each is designed for its span and load. A truss can carry a given load over a longer span with less lumber, but only as built. Damage or cutting can compromise the whole frame.
Can you cut a roof truss?
Not without approval from the truss designer or a licensed structural engineer, who will provide a repair or modification detail. Trimming overhang tails to fit the fascia is normal; cutting chords or webs is not.
How do I tell if my house has trusses or rafters?
Look in the attic. Trusses have diagonal web pieces between the top and bottom members, with metal plates at the joints, repeated identically at every framing location. Rafters are single sloped boards meeting at a ridge, with separate ceiling joists below and an open attic.
Are floor trusses better than I-joists?
They suit different needs. Floor trusses make it easy to run ducts and pipes through the floor and can span long distances. I-joists can be cut to length on site and are stocked by lumber yards. Pick based on span, mechanical layout and schedule.
Can I hang a load from a truss bottom chord?
Only up to what the truss was designed for, which is shown on the truss design drawings. For anything heavier than ordinary ceiling finishes, have a licensed structural engineer or the truss manufacturer check it.
Reference information only, not engineering advice. Disclaimer.